Reinforced polymer x-ray window

ABSTRACT

An x-ray window comprising a polymer and carbon nanotubes and/or graphene. The carbon nanotubes and/or graphene can be embedded in the polymer. Multiple layers of polymer, carbon nanotubes, and/or graphene may be used. The polymer with carbon nanotubes and/or graphene can be used as an x-ray window support structure and/or thin film.

CLAIM OF PRIORITY

This is a continuation-in-part of U.S. patent application Ser. No.12/239,281, filed on Sep. 26, 2008; which claims priority of U.S. PatentApplication Ser. No. 60/995,881, filed Sep. 28, 2007; and is also acontinuation-in-part of U.S. patent application Ser. No. 12/899,750,filed Oct. 7, 2010; which are hereby incorporated by reference.

This also claims priority to U.S. Provisional Patent Application Ser.No. 61/437,792, filed on Jan. 31, 2011; which is incorporated byreference.

BACKGROUND

X-ray windows are used for enclosing an x-ray source or detectiondevice. The window can be used to separate air from a vacuum within theenclosure while allowing passage of x-rays through the window.

X-ray windows can be made of a thin film. It can be desirable tominimize attenuation of the x-rays, especially with low energy x-rays,thus it is desirable that the film is made of a material and thicknessthat will result in minimal attenuation of the x-rays. Thinner filmsattenuate x-rays less than thick films, but the film must not be toothin or the film may sag or break. A sagging film can result in crackingof corrosion resistant coatings and a broken film will allow air toenter the enclosure, often destroying the functionality of the device.Thus it is desirable to have a film that is made of a material that willhave sufficient strength to avoid breaking or sagging but also as thinas possible for minimizing attenuation of x-rays.

A support structure can be used to support the thin film. Use of asupport structure can allow use of a thinner film than could be usedwithout the support structure. For example, a support structure can bemade of a plurality of ribs with openings therein. The thin film can beattached to and span the ribs and openings. In order to minimizeattenuation of x-rays, it is desirable that the ribs of the structurehave a smaller width and height. Wider and higher ribs are typicallystronger. Stronger rib materials can provide sufficient strength at asmaller size.

X-ray windows are often used with x-ray detectors. In order to avoidcontamination of an x-ray spectra from a sample being measured, it isdesirable that x-rays impinging on the x-ray detector are only emittedfrom the source to be measured. Unfortunately, x-ray windows, includingthe window support structure and thin film, can also fluoresce and thusemit x-rays that can cause contamination lines in the x-ray spectra.Contamination of the x-ray spectra caused by low atomic number elementsis less problematic than contamination caused by higher atomic numberelements. It is desirable therefore that the window and supportstructure be made of a material with as low of an atomic number aspossible in order to minimize this noise.

SUMMARY

It has been recognized that it would be advantageous to have an x-raywindow that is strong, minimizes attenuation of x-rays, and minimizesx-ray spectra contamination. The present invention is directed to anx-ray window that satisfies the need for an x-ray window that is strong,minimizes attenuation of x-rays, and minimizes x-ray spectracontamination.

In one embodiment, the x-ray window includes a film comprised of apolymer and a high strength material. The high strength materialcomprises carbon nanotubes and/or graphene. The high strength materialreinforces the polymer, thus making a stronger polymer layer. Carbon hasa low atomic number (6) and thus is less likely to contaminate an x-rayspectra than an element with a higher atomic number.

In another embodiment, the x-ray window includes a plurality of ribshaving openings and a support frame disposed around and connected to aperimeter of the ribs. The ribs and the support frame comprise a highstrength material and a The high strength material reinforces thepolymer. A thin film is disposed over and spans the plurality of ribsand openings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional side view of an x-ray window filmincluding a high strength material and a polymer in accordance with anembodiment of the present invention;

FIG. 2 is a schematic cross-sectional side view of an x-ray windowcomposite film including a high strength material embedded in a polymerin accordance with an embodiment of the present invention;

FIG. 3 is a schematic cross-sectional side view of an x-ray windowcomposite film including a high strength material embedded in a polymerand also additional polymer layers disposed adjacent to the compositefilm, in accordance with an embodiment of the present invention;

FIG. 4 is a schematic cross-sectional side view of an x-ray window filmincluding carbon nanotubes embedded in a polymer and a majority of thecarbon nanotubes are aligned substantially parallel with respect to asurface of the film, in accordance with an embodiment of the presentinvention;

FIG. 5 is a schematic cross-sectional side view of an x-ray window filmincluding carbon nanotubes embedded in a polymer and a majority of thecarbon nanotubes are randomly aligned, in accordance with an embodimentof the present invention;

FIG. 6 is a schematic cross-sectional side view of an x-ray window filmincluding two layers of high strength material and two layers of polymerin accordance with an embodiment of the present invention;

FIG. 7 is a schematic cross-sectional side view of two adjacent x-raywindow composite films, each including a high strength material embeddedin a polymer, in accordance with an embodiment of the present invention;

FIG. 8 is a schematic cross-sectional side view of an x-ray windowincluding a thin film and a support structure accordance with anembodiment of the present invention;

FIG. 9 is a schematic cross-sectional side view of an x-ray tubeincluding an x-ray window in accordance with an embodiment of thepresent invention;

FIG. 10 is a schematic cross-sectional side view of an x-ray detectorincluding an x-ray window in accordance with an embodiment of thepresent invention;

FIG. 11 is a scanning electron microscope image of a vertically alignedcarbon nanotube layer in accordance with an embodiment of the presentinvention;

FIG. 12 is a scanning electron microscope image of a vertically alignedcarbon nanotube layer, the carbon nanotubes having a height of about 2.4μm, in accordance with an embodiment of the present invention; and

FIG. 13 is a scanning electron microscope image of a carbon nanotubelayer, after rolling the carbon nanotubes flat, in accordance with anembodiment of the present invention.

DEFINITIONS

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint.

As used herein, the term “CNT” means carbon nanotubes or carbonnanotube.

As used herein, the term “sccm” means standard cubic centimeters perminute.

As used herein, the term “substantially” refers to the complete ornearly complete extent or degree of an action, characteristic, property,state, structure, item, or result. For example, an object that is“substantially” enclosed would mean that the object is either completelyenclosed or nearly completely enclosed. The exact allowable degree ofdeviation from absolute completeness may in some cases depend on thespecific context. However, generally speaking the nearness of completionwill be so as to have the same overall result as if absolute and totalcompletion were obtained. The use of “substantially” is equallyapplicable when used in a negative connotation to refer to the completeor near complete lack of an action, characteristic, property, state,structure, item, or result.

As used herein, the term “VACNT” means vertically aligned carbonnanotubes.

DETAILED DESCRIPTION

Reference will now be made to the exemplary embodiments illustrated inthe drawings, and specific language will be used herein to describe thesame. It will nevertheless be understood that no limitation of the scopeof the invention is thereby intended. Alterations and furthermodifications of the inventive features illustrated herein, andadditional applications of the principles of the inventions asillustrated herein, which would occur to one skilled in the relevant artand having possession of this disclosure, are to be considered withinthe scope of the invention.

As illustrated in FIG. 1, an x-ray window film 10 is shown comprising apolymer 12 and a high strength material 11. The high strength material11, in the various embodiments described herein, can comprise carbonnanotubes, graphene, or combinations thereof. The film 10 can have athickness T1 of between about 50 nm to about 500 nm. Addition of a highstrength material 11 to a polymer 12 can provide for a higher strengthfilm 10 than with a polymer alone, thus allowing the film 10 to spangreater distances without breaking or sagging.

A high strength material 11 can be an individual layer or may beembedded in the polymer 12 as shown in the x-ray window film 20 of FIG.2. The film 20 can have a thickness T2 of between about 50 nm to about500 nm. As illustrated in FIG. 3, an x-ray window film 30 is showncomprising a composite film including a high strength material 11embedded in a polymer 12 and further comprising polymer layers 32 a-bdisposed adjacent to the composite film. The film 30 can have athickness T3 of between about 50 nm to about 500 nm.

The high strength material 11, in the various embodiments describedherein, can include carbon nanotubes. As illustrated in FIG. 4, in x-raywindow 40 a majority of the carbon nanotubes 41 can be alignedsubstantially parallel with respect to a surface of the film 42. Asdescribed in more detail later, this substantially parallel alignmentmay be accomplished by rolling the carbon nanotubes 41 flat with use ofa roller that is not very much larger in diameter than a height of thecarbon nanotubes 41. As illustrated in x-ray window 50 of FIG. 5, amajority of the carbon nanotubes 51 can be randomly aligned. Asdescribed in more detail later, this random alignment may beaccomplished by rolling the carbon nanotubes 51 flat with use of aroller that is very much larger in diameter than a height of the carbonnanotubes 51 or by spraying the carbon nanotubes onto a surface, such asa polymer.

As illustrated in FIG. 6, x-ray window 60 can include at least twolayers of high strength material 11 a-b and at least two layers ofpolymer material 12 a-b. In another embodiment, the x-ray window caninclude at least three layers of high strength material and at leastthree layers of polymer material. The high strength material layers andpolymer layers can alternate. The x-ray window film 60 can have athickness T6 of between about 50 nm to about 500 nm.

As illustrated in FIG. 7, x-ray window 70 can include at least twolayers of high strength material 11 c-d and at least two layers ofpolymer material 12 c-d. In another embodiment, the x-ray window caninclude at least three layers of high strength material and at leastthree layers of polymer material. As illustrated in FIG. 7, the highstrength material 11 c-d can be embedded in the polymer 12 c-d. Thex-ray window film 70 can have a thickness T7 of between about 50 nm toabout 500 nm.

In the various embodiments described herein, the polymer can comprise apolyimide. In the various embodiments described herein, the x-ray windowfilm, comprising polymer and high strength material, can besubstantially transmissive to x-rays having an energy in the range of100-20,000 electronvolts; can be capable of withstanding a differentialpressure of at least 1 atmosphere; and/or can be capable of withstandingtemperatures of greater than 225° C. Materials and thicknesses may beselected to allow the window to withstand a differential pressure of atleast 1 atmosphere, thus allowing the window to be used in a device,such as an x-ray detector or x-ray tube, with vacuum on one side, andatmospheric pressure on the other side. Materials may be selected toallow the window to withstand temperatures of greater than 225° C.Sometimes there is a need to subject x-ray windows to highertemperatures, such as in manufacturing, thus it can be valuable to havean x-ray window that can withstand high temperatures.

As illustrated in FIG. 8, x-ray window 80 can include a film 86supported by a support structure 87. The support structure can comprisea plurality of ribs 83 having openings 84 therein, wherein tops of theribs 83 terminate substantially in a common plane. The support structure87 can also include a support frame 85 disposed around a perimeter ofthe plurality of ribs 83. The film 86 can be disposed over and span theplurality of ribs 83 and openings 84. The support structure 87 can givesupport to the film 86, thus allowing the film 86 to span largerdistances without sagging. In one embodiment, the film 86 comprises apolymer and a high strength material according to the variousembodiments described herein. In another embodiment, the supportstructure 87 comprises a polymer and a high strength material accordingto the various embodiments described herein, and the film 86 comprisesdiamond, graphene, diamond-like carbon, carbon nanotubes, polymer,beryllium, or combinations thereof. In one embodiment, the openings takeup about 70% to about 95% of a total area within an inner perimeter ofthe support frame. A larger area for openings can be desirable forminimizing attenuation of x-rays in the support structure 87. In oneembodiment, each rib is about less than 100 μm wide w and each rib isbetween about 30 μm to about 300 μm high h.

As illustrated in FIG. 9, x-ray window embodiments described herein canbe mounted on an x-ray tube 90. The x-ray tube can comprise an evacuatedcylinder 82, a cathode 83 disposed at one end of the evacuated cylinder;and a window and anode 81 disposed at an opposing end of the cylinder82.

As illustrated in FIG. 10, x-ray window embodiments described herein canbe used for x-ray detection 100. The x-ray window 101 can be attached toa mount 102. The mount 102 can be attached to an x-ray detector 103; andthe x-ray window 101 can allow x-rays 104 to impinge upon the detector103.

How to Make: Carbon Nanotube Formation:

A carbon nanotube film may be formed by placing a substrate with a layercomprising alumina and a layer comprising iron in an oven at atemperature of greater than 600° C. then flowing ethylene across thesubstrate thus allowing carbon nanotubes to grow on the substrate.Growth rate can be controlled by the ethylene flow rate and by dilutingthe ethylene with argon gas. Thickness of the carbon nanotube forest canbe controlled by the ethylene flow time. Use of sputtered iron catalystinstead of thermal deposited iron can result in slower carbon nanotubegrowth.

An example of one method of forming the carbon nanotubes: A siliconwafer was coated with a 30 nm alumina layer A 6 nm iron layer was thendeposited on the alumina layer by PVD sputtering. CNT forest sampleswere made having thicknesses of around 2 μm, 1 μm, and 500 nm with 1second ethylene flow with different ethylene flow rates.

Samples were put onto a quartz boat and loaded into a quartz tube of atube furnace (CNT growth furnace). Argon was switched on to flow intothe tube furnace at 50% flow rate (355 sccm) and kept on during thewhole growth cycle. After Argon purged the air out of the tube, hydrogenflow was turned on at a 20% flow rate (429 sccm) and the tube furnacewas heated up to 750° C.

Ethylene flow was turned on for 1 second for short CNT forest growth at50% flow rate (604 sccm). Shorter forests were produced with lowerethylene flow rate. Ethylene and hydrogen flow were turned offimmediately after the one second growth.

The cover of the tube furnace was opened to accelerate the coolingprocess. When the temperature was down to 200° C., the samples weretaken out from the tube furnace. Argon flow was turned off. This CNTgrowth cycle was finished.

Combining Carbon Nanotubes with Polymer—Method 1:

The carbon nanotubes can be aligned horizontally, or aligned randomly,by placing a film on top of the carbon nanotubes, rolling the carbonnanotubes flat with a cylindrical roller, then removing the film. Foralignment of the carbon nanotubes in substantially a single direction,or in a direction substantially parallel with a surface of the film, theroller should not be very much larger in diameter than a height of thecarbon nanotubes. Rollers that are much larger than the diameter of theroller can result in more random alignment of the carbon nanotubes.

For Example of One Method of Rolling the Carbon Nanotubes:

A VACNT forest sample with a size of around 18 mm×18 mm was directlyplaced on a flat, hard desk surface to avoid substrate cracking. Analuminum foil of about 30 mm×30 mm was placed over and covered the wholenanotube sample surface. Tape was used to cover the edges of thesubstrate and aluminum foil to avoid substrate shift. A 50 mm×80 mmnitrile sheet of about 0.4 mm in thickness was placed over the aluminumfoil and also taped to the desk. A smooth glass tube with 1.57 cm outerdiameter was rolled and pressed over the nitrile rubber sheet, aluminumfoil, and the CNT sample from different directions for 100 times. Thenitrile rubber sheet and the aluminum foil were removed. A thinner anddenser CNT film was obtained.

A polymer film can then be applied, such as by placing a polymer film onthe carbon nanotubes. The polymer film may be pressed onto the carbonnanotubes in order to embed the carbon nanotubes in the film.Alternatively, a liquid polymer may be poured onto the carbon nanotubesor spun onto the carbon nanotubes, then the polymer can harden bysuitable method. The carbon nanotubes can then be released from thesubstrate, such as by use of hydrofluoric acid.

Combining Carbon Nanotubes with Polymer—Method 2:

Carbon nanotubes may be sprayed onto a polymer film. Alternatively,carbon nanotubes may be sprayed onto a liquid polymer, then the polymermay be cured. A method for spraying carbon nanotubes is described inChemical Engineering Science, “Insights into the physics of spraycoating of SWNT films”, available online 5 Dec. 2009, which isincorporated herein by reference.

In summary of the above method, a suspension of carbon nanotubes may beprepared by an appropriate solvent, such as water with a surfactant, andsonication. The carbon nanotube suspension may then be sprayed onto theappropriate surface. In the present invention, the carbon nanotubesuspension can be sprayed onto a polymer. Another polymer layer can bedeposited onto the carbon nanotubes, such as by spin coating.

Layer Including Graphene:

A graphene film may be made by flowing methane across a copper surfacein an oven at a temperature of greater than 1000° C., thus allowingformation of a graphene layer. The copper may be removed from thegraphene layer such as by dissolving the copper in an acid. Liquidpolymer may be sprayed on or poured on then cured, such as in an oven,thus forming a composite layer with graphene and polymer.

Other Manufacturing Issues:

The above methods may be combined for making a film with graphene,carbon nanotubes, and polymer. Multiple layers of carbon nanotube andpolymer may be stacked together. Multiple layers of graphene and polymermay be stacked together. A layer, or layers, carbon nanotube and polymermay be stacked with a layer, or layers, of graphene and polymer.

A support structure can be made by patterning and etching. The supportstructure can be made of polymer and a high strength material, or may bemade of other material. A film, or layers of films may be placed ontothe support structure. The film can comprise diamond, graphene,diamond-like carbon, carbon nanotubes, polymer, beryllium, orcombinations thereof. An adhesive may be used to adhere the film to thesupport structure.

It is to be understood that the above-referenced arrangements are onlyillustrative of the application for the principles of the presentinvention. Numerous modifications and alternative arrangements can bedevised without departing from the spirit and scope of the presentinvention. While the present invention has been shown in the drawingsand fully described above with particularity and detail in connectionwith what is presently deemed to be the most practical and preferredembodiment(s) of the invention, it will be apparent to those of ordinaryskill in the art that numerous modifications can be made withoutdeparting from the principles and concepts of the invention as set forthherein.

1. An x-ray window including a film, the film comprising: a) a polymerand a high strength material; and b) the high strength materialcomprising a material selected from the group consisting of carbonnanotubes, graphene, and combinations thereof.
 2. The x-ray window ofclaim 1, wherein the film has a thickness of between about 50 nm toabout 500 nm.
 3. The x-ray window of claim 1, wherein the film includesa composite film comprising the high strength material embedded in thepolymer.
 4. The x-ray window of claim 3, further comprising at least onepolymer layer disposed adjacent to the composite film.
 5. The x-raywindow of claim 1, wherein the high strength material includes carbonnanotubes, and a majority of the carbon nanotubes are alignedsubstantially parallel with respect to a surface of the film.
 6. Thex-ray window of claim 1, wherein the high strength material includescarbon nanotubes, and a majority of the carbon nanotubes are randomlyaligned.
 7. The x-ray window of claim 1, wherein the film comprises atleast two films stacked together, each film including the high strengthmaterial and the polymer.
 8. The x-ray window of claim 1: a) furthercomprising a plurality of ribs having openings therein, wherein tops ofthe ribs terminate substantially in a common plane, and a support framedisposed around a perimeter of the plurality of ribs; and b) wherein thefilm is disposed over and spans the plurality of ribs and openings topass radiation therethrough.
 9. The x-ray window of claim 1, wherein thepolymer comprises a polyimide.
 10. The x-ray window of claim 1, wherein:a) the film is substantially transmissive to x-rays having an energy inthe range of 100-20,000 electronvolts; b) the film is capable ofwithstanding a differential pressure of at least 1 atmosphere; and b)the film is capable of withstanding temperatures of greater than 225° C.11. The x-ray window of claim 1, wherein the film is mounted on an x-raytube.
 12. The x-ray window of claim 1, wherein: a) the film is attachedto a mount; b) the mount is attached to an x-ray detector; and c) thefilm allow x-rays to impinge upon the detector.
 13. The x-ray window ofclaim 1, wherein: a) the polymer includes at least two polymer layers;b) the high strength material includes at least two high strengthmaterial layers; and c) at least two of the at least two high strengthmaterial layers are separated by at least one polymer layer.
 14. Anx-ray window comprising: a) a plurality of ribs having openings thereinand tops of the ribs terminate substantially in a common plane; b) asupport frame disposed around and connected to a perimeter of theplurality of ribs; c) the ribs and support frame comprise a highstrength material and a polymer; d) the high strength materialcomprising a material selected from the group consisting of carbonnanotubes, graphene, and combinations thereof; and d) a thin filmdisposed over and spanning the plurality of ribs and openings to passradiation therethrough.
 15. The x-ray window of claim 14 wherein theopenings take up about 70% to about 95% of a total area within an innerperimeter of the support frame.
 16. The x-ray window of claim 14 whereineach rib is about less than 100 μm wide and each rib is between about 30μm to about 300 μm high.
 17. The x-ray window of claim 14 wherein thethin film comprises a material selected from the group consisting ofdiamond, graphene, diamond-like carbon, carbon nanotubes, polymer,beryllium, and combinations thereof.
 18. The x-ray window of claim 14wherein the high strength material is embedded in the polymer.
 19. Anx-ray window comprising: a) a composite film comprising a high strengthmaterial embedded in a polymer; b) the high strength material comprisinga material selected from the group consisting of carbon nanotubes,graphene, and combinations thereof; c) the composite film has athickness of between about 50 nm to about 500 nm; d) the composite filmis substantially transmissive to x-rays having an energy in the range of100-20,000 electronvolts; e) the composite film is capable ofwithstanding a differential pressure of at least 1 atmosphere; and f)the composite film is capable of withstanding temperatures of greaterthan 225° C.
 20. The x-ray window of claim 19, wherein the high strengthmaterial includes carbon nanotubes, and a majority of the carbonnanotubes are randomly aligned.